This invention relates generally to PCM signal processing circuits, and particularly to a data acquisition circuit of a PCM processor used for recording and reproducing audio signals by using video tape recorders.
It has been several years since PCM (pulse code modulation) processors were put on market, which are to be connected to a home-use video tape recorder (which will be referred to as VTR hereinafter) using a 1/2 inch magnetic recording tape for recording and reproducing audio signals in order to effect new high-fidelity recording and reproducing of audio signals. Although home-use VTRs were of standard-time mode at the time of designing such PCM processors, today's home-use VTRs are mainly of long-time mode, in which the tape motion speed is made lower than that in standard-time mode, because of recent technical innovation such as high-density recording, progress in reproducing techniques, and improvement in magnetic recording tapes. Under the above-mentioned condition, PCM processors, which are designed to be used in standard-time mode where recording and reproducing characteristics are satisfactory, are sometimes used in long-time mode of home-use VTRs. However, since recording and reproducing characteristics in long-time mode is not satisfactory compared to standard-time mode, some troubles would occur due to the occurrence of noise and operation of a muting circuit.
PCM processors, which record and reproduce audio signals by using a home-use VTR are described in detail in a technical file (STC-007,008) or in a standard CPZ-105 of EIAJ (Electronic Industry Association of Japan), and PCM processors now on the market are all designed according to the recording format (which may be referred to as EIAJ format) desribed in the EIAJ technical file or standard. In addition, various materials are disclosed as to brief description of PCM processors according to EIAJ format. Therefore, the present description is provided mainly in connection with a data acquisition circuit which extracts digital data from a reproduced signal from a VTR.
Although a conventional data acquistion circuit used in a PCM processor satisfactorily operates when a PCM signal is reproduced from a VTR which is operated in standard-time mode, the conventional data acquisition circuit cannot be satisfactorily used when a PCM signal is reproduced from a VTR operated in long-time mode because of deterioration or distortion in the waveform of the reproduced PCM signal.